A number of different oligomerisation technologies are known to produce α-olefins. Some of these processes, including the Shell Higher Olefins Process and Ziegler-type technologies, have been summarized in WO 04/056479 A1. The same document also discloses that the prior art (e.g. WO 03/053891 and WO 02/04119) teaches that chromium based catalysts containing heteroaromatic ligands with both phosphorus and nitrogen heteroatoms, selectively catalyse the trimerisation of ethylene to 1-hexene.
Processes wherein transition metals and heteroaromatic ligands are combined to form catalysts for trimerisation, tetramerisation, oligomerisation and polymerisation of olefinic compounds have also been described in different patent applications such as WO 03/053890 A1; WO 03/053891; WO 04/056479 A1; WO 04/056477 A1; WO 04/056480 A1; WO 04/056478 A1; WO 05/123884 A2; WO 05/123633 A1 and U.S. Pat. No. 7,285,607.
The catalysts utilized in the abovementioned trimerisation, tetramerisation, oligomerisation or polymerisation processes all include one or more activators to activate the catalyst. Suitable activators include organoaluminium compounds, organoboron compounds, organic salts, such as methyl lithium and methyl magnesium bromide, inorganic acids and salts, such as tetrafluoroboric acid etherate, silver tetrafluoroborate, sodium hexafluoroantimonate and the like.
A common catalyst activator used in combination with Cr based catalysts for oligomerisation of olefinic compounds is alkylaluminoxane, particularly methylaluminoxane (MAO). It is well known that MAO includes significant quantities of alkylaluminium in the form of trimethylaluminium (TMA), and in effect the catalyst activator is a combination of TMA and MAO. The MAO may also be replaced with modified MAO (MMAO), which may contain free trialkylaluminium in the form of TMA and heavier trialkylaluminiums. The use of organoboron compounds as catalyst activators is also known.
Activators containing aluminium compounds are costly to the effect that it impacts significantly on process economics of olefin oligomerisation technologies that utilize this class of activators. For this reason, it is desirable to run commercial oligomerisation processes at low activator concentrations. However, in the case where an aluminium-containing compound was used as an activator for transition metal based oligomerisation catalysts, it was found that at conditions of low starting aluminium concentrations (e.g. <6 mmol/l), low reaction rates and high levels of unwanted solid formation (polyethylene (PE) and waxes) resulted when ethylene was oligomerised.
Reduction in the formation of polymers as a by-product in Cr-based ethylene oligomerisation (both tri- and tetramerisation) processes remains an ongoing challenge, as polymer fouling reduces plant run time and necessitates shut-downs due to blockages. Furthermore, high catalyst activity must accompany low polymer formation in order that good space-time yields are obtained, and high catalyst productivity is required in order to maintain acceptable economic performance.
Generally speaking, the literature teaches towards the removal of impurities, specifically oxygen, from the system before ethylene polymerisation or oligomerisation catalysis is performed (see US2010/0081777A1 and US2011/0282016A1 for example) as oxygen is considered a poison (see M. P Daniels and S. J. Martin, J. Phys. Chem. 1991, 95, 3289-3293).